Here are some photos of Tony Trabert's shoulder turn on the serve. I used to read that you should almost show your back to your opponent.
Lots of pros today do not do it though. Wawrinka doesn't seem to do it, for example. Points his chest towards the ball, but very little upper body torsion.
Seems to me that this torsion adds speed to the kinetic chain.
Maybe this is more a charateristic of the retro serve when you were not allowed to jump?
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